Fe-based amorphous alloy ribbons have been attracting much attention for magnetic cores for transformers because of excellent soft magnetic properties, particularly low core loss. Particularly amorphous Fe—Si—B alloy ribbons having high saturation magnetic flux densities BS and excellent thermal stability are used for magnetic cores for transformers. However, the Fe-based amorphous alloy ribbons are poorer than silicon steel plates presently used mostly for magnetic cores for transformers in saturation magnetic flux density. Thus, development has been conducted to provide Fe-based amorphous alloy ribbons with high saturation magnetic flux densities. To increase the saturation magnetic flux density, various attempts have been conducted: the amount of Fe contributing to magnetization is increased; the decrease of thermal stability due to increase in the amount of Fe is compensated by adding Sn, S, etc.; and C is added.
JP 5-140703 A discloses an amorphous Fe—Si—B—C—Sn alloy having a high saturation magnetic flux density, in which Sn serves to make the high-Fe-content alloy amorphous. JP 2002-285304 A discloses an amorphous Fe—Si—B—C—P alloy having a high saturation magnetic flux density, in which P serves to make the alloy having a drastically increased Fe content amorphous.
It is important that practical magnetic cores have a high magnetic flux density at a low magnetic field, namely a high squareness ratio B80/BS, in which B80 represents a magnetic flux density in a magnetic field of 80 A/m. What is practically important for magnetic cores for transformers is that the transformers are operated at a high magnetic flux density. The operating magnetic flux density is determined by the relation between a magnetic flux density and a core loss, and should be lower than the magnetic flux density from which the core loss increases drastically. Even with the same saturation magnetic flux density, Fe-based amorphous alloy ribbons having low B80/BS would have increased core losses at high operating magnetic flux densities. In other words, Fe-based amorphous alloy ribbons having higher B80 and lower core losses in high magnetic flux density regions can be operated at higher operating magnetic flux densities. However, Fe-based amorphous alloy ribbons having B80 of more than 1.55 T are not mass-produced at present. The reason therefor is that if alloy ribbons having high saturation magnetic flux densities contain more than 81 atomic % of Fe, they cannot be mass-produced stably because of surface crystallization and thermal stability decrease. To solve such problems, attempts have been conducted to improve surface crystallization and thermal stability by adding Sn, S, etc. Though these means can improve alloy's properties, the resultant ribbons are brittle, and ribbons having additives distributed uniformly cannot be produced continuously. For these reasons, such amorphous alloy ribbons cannot be mass-produced. Though C-containing alloys having an Fe content of 81 atomic % can be mass-produced, they have B80 of 1.55 T or less. In addition, embrittlement, surface crystallization and thermal stability decrease are serious problems for Fe-based amorphous alloy ribbons containing 81 atomic % or more of Fe. Though the addition of C and P can improve saturation magnetic flux densities, the resultant ribbons are so brittle that they cannot be easily formed into transformers.
As described above, despite the effort of improving the saturation magnetic flux densities of Fe-based amorphous alloy ribbons, Fe-based amorphous alloy ribbons having B80 of 1.55 T or more and core losses W14/50 of 0.28 W/kg or less when measured on toroidal cores have not been stably produced so far, because of embrittlement, surface crystallization and squareness ratio decrease, etc.